The present invention relates to an electronic balance in which an improved Roberval mechanism is used.
An electronic balance of a type in which the weighing pan is positioned above the weight receiving mechanism has that weight receiving mechanism constituted in the form of a Roberval mechanism. The Roberval mechanism is a parallelogram consisting of four side members linked by means of pivots or elastic joints. Two of the four side members are kept vertical with one fixed so as not to move and with the other prolonged upward to support the weighing pan. In many cases the fixed vertical side member is replaced by a vertical column-like member integrated with or fixed to the base frame of the balance. The other (not fixed) vertical side member supporting the weighing pan is accompanied by a lever mechanism for transmitting a weight force given on the weighing pan to an electronic weight sensor. Thus the weight loaded on the weighing pan is transmitted to the weight sensor through the Roberval and the lever mechanisms.
However, since such a weight force transmitting system consists of a plurality of separate consistuents such as the lever mechanism, the four side members of the Roberval mechanism, and the pivots or the elastic joints, the system is made very complicated. In addition these constituents are in general made of different materials; the Roberval mechanism and the lever are, for example, made of die-cast aluminum alloy, while the joint members such as the pivots and elastict joints are of steel. These constituents are assembled to form the weight force transmitting system by using bolts, nuts, adhesives and other suitable connecting or fixing means. Therefore, the difference in thermal expansion coefficient among the materials used may cause, when the ambient temperature varies, the system to be distorted and, as a result, may give variations to the friction at the pivotally jointed portions and to the elasticity of the elastic joints. The conventional electronic balance is thus accompanied by the risk that the precision and stability are affected by the ambient temperature variations.